276 Electrical Power Systems Technology
Find: the voltage across lamp number 4.
Solution:
- Find the resistance for 20 feet (6.1 meters) of conductor (same as for
10-foot conductor × 2). No. 14 copper wire is used for 15-ampere
branch circuits. From Table 10-2, we find that the resistance of 1000
feet (304.8 meters) of No. 14 copper wire is 2.57 ohms. Therefore, the
resistance of 20 feet of wire is:
20 feet R
———— = ————
1000 feet 2.57 ohms
1000 R = (20) (2.57)
R = 0.0514 ohm.
- Calculate voltage drop VD number 1. (R equals the resistance of 20
feet of wire.)
VD number 1 = I × R
= 12 A × 0.0514 Ω
= 0.6168 V AC
Calculate load V 1 , number 1 (the source voltage minus VD number
1.)
VL number 1 = 120 V – 0.6168 V
= 119.383 volts
Figure 10-7. Circuit for calculating the voltage drop in a branch circuit